A complex international operation to free a Russian ship trapped in Antarctica ended Wednesday when the vessel cleared the ice field, as officials released a United States icebreaker from the rescue.

The Russian-flagged Akademik Shokalskiy became stuck on December 24, triggering a rescue operation coordinated by Australian authorities which included French, Chinese, American and German agencies.

China's powerful icebreaker Xue Long raced to respond to the stranded ship, which had been carrying 74 scientists, passengers and crew, but was unable to break through and became trapped itself.

The Australian Maritime Safety Authority (AMSA) confirmed Wednesday that the Akademik Shokalskiy and the Xue Long "have broken free from the ice in Antarctica and are no longer in need of assistance".

The United States Coast Guard 399-foot long icebreaker Polar Star, which had been en route to the stranded ships, was released from the rescue.

It will now continue on its original mission of resupplying and refuelling the US's McMurdo and Amundsen-Scott South Pole stations during the weather window provided by the southern hemisphere summer.

"We are extremely pleased to learn that both the Xue Long and the Akademik Shokalskiy freed themselves from the ice," US Coast Guard commander Vice Admiral Paul F. Zukunft said.

"This case underscores the dynamic and harsh operating environment and the necessity for Polar Class Icebreakers in the Antarctic."

AMSA said the captain of the Russian ship Igor Kiselyov told officials on Tuesday that cracks had started to open in the ice around the trapped vessel and a short time later it began to make slow movements.

"Finally the wind changed to the west and as a result a crack appeared in the ice," Kiselyov told Russia's ITAR-TASS news agency. "We went into it and we are now slowly moving north," he said late Tuesday.

The ship quickly managed to clear the area containing the heaviest ice and despite the hard conditions, including thick fog and reduced visibility, began making slow progress north through lighter ice conditions.

The ship, which has 22 crew onboard, took about 12 hours to leave the ice field behind and is now heading to New Zealand, AMSA said.

The captain of the Chinese vessel had also managed to break free of the heaviest ice around the same time "and is now making slow progress through lighter ice conditions" and is in need of no assistance, it said.

The ice surrounding the Xue Long, which has 101 people on board, had been up to four metres (13 feet) thick.

The Xue Long, or Snow Dragon, had been trapped since Friday, one day after dramatically rescuing 52 scientists, passengers and journalists from the Shokalskiy using its helicopter.

The 52 were transferred to the Australian Antarctic programme's supply ship the Aurora Australis, which also raced to the rescue but was unable to break through the ice to reach the Russian vessel.

In total five ships were involved in the search and rescue mission -- Akademik Shokalskiy, the French vessel L'Astrolabe, Xue Long, Aurora Australis and USCGC Polar Star.

The national Antarctic programmes and other agencies of France, China, Australia, Germany and the US helped with "actual operational responses, contingency planning or the provision of specialist data", AMSA said.

"This was a great example of the multi-lateral cooperative nature of Antarctic operations," the authority's acting chief executive Mick Kinley said.

But the diversion of several government's Antarctic programme's resources to help the Shokalskiy has drawn criticism of its "Spirit of Mawson" expedition which was emulating a 1911-1914 tour by explorer Sir Douglas Mawson.

The Aurora, which was resupplying Australia's Antarctic base at Casey when it was diverted, is now running two weeks behind schedule, while French scientists have had to scrap a two-week oceanographic campaign.

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